As an example of the energy management system, in recent vehicles, a micro hybrid electric vehicle (hereinafter, referred to as a micro HEV) has been developed with a function that, in addition to an idling stop function, converts a regenerative energy during a deceleration into an electrical energy by a generator (alternator) to charge a battery so that the battery is used as a power source of auxiliary equipment such as headlights and heater which is an electrical load. Here, a lead acid battery and another type of battery (hereinafter, referred to as a sub battery) are sometimes used as the secondary battery of the recent micro HEV. This is in order to recover more regenerative energy. As for the sub battery used in three types of vehicles, an open circuit voltage (hereinafter referred to as OCV) is substantially the same as that of the lead acid battery. Thus, it is possible to prevent a current exchange between the batters (hereinafter referred to as a cross current), even when the two batteries of the lead acid battery and the sub battery are connected in parallel.
However, when a battery with different OCV from the lead acid battery is used for the sub battery, a cross current occurs, resulting in a loss and thus unable to recover a sufficient regenerative energy. In particular, the cross current is significant when a capacitor (although not exactly a battery but an electrical storage device, this will be referred to as a battery in this specification) with an excellent temperature resistance and lifetime that can increase the charging current is used for the sub battery.
In order to prevent the cross current, there is a method for inserting a DC-to-DC converter between the lead acid battery and the sub battery. However, this method leads to higher costs. Thus, to achieve lower costs, as a hard configuration (not limited to the micro HEV), there may be considered a method or preventing the cross current by inserting a switch SW into each of the lead acid battery and the sub battery in series to connect the batteries in parallel, in accordance with the method described in Patent Document 1 (Japanese Unexamined Patent Application Publication No. 2010-115050).
As the switch SW switching method, Patent Document 1 discloses a method of switching the switch SW so that the charge rate of the main battery and the charge rate of the sub battery are even with each other. Further, Patent Document 2 discloses a switching method by a voltage variation.